1. Field of the Invention
The present invention relates to forming electrical contacts on thin-film semiconductor devices. "Thin-film" semiconductor devices generally are understood to comprise layers of material each less than 10 micrometers (100,000 .ANG.) thick sucessively fabricated on a flat substrate or superstrate. More particularly, the present invention relates to forming back contacts on photovoltaic cells comprised of thin films of amorphous silicon.
2. Description of the Related Art
As is well known in the thin-film semiconductor art, photovoltaic cells that convert solar radiation into usable electrical energy can be fabricated by sandwiching certain semiconductor structures, such as, for example, the amorphous silicon PIN structure disclosed in U.S. Pat. No. 4,064,521, between two electrical contacts. One of the contacts typically is transparent to permit solar radiation to reach the semiconductor material. This "front" contact can be comprised of a thin film (i.e., less than 10 micrometers) of transparent conductive oxide material such as tin oxide and usually is formed between a transparent supporting superstrate made of glass or plastic and the photovoltaic semiconductor material. The "back" contact, which is formed on the surface of the semiconductor material opposite the front contact, generally comprises a thin film of metal such as, for example, aluminum.
The voltage produced across the contacts of a single-cell photovoltaic module, however, is insufficient for most applications. To achieve a useful power level from photovoltaic semiconductor devices, individual photovoltaic cells must be electrically connected in series. A typical arrangement of series-connected photovoltaic cells is shown in FIG. 1.
FIG. 1 shows photovoltaic module 10 comprised of a plurality of series-connected photovoltaic cells 11 formed on a transparent superstrate 12 and subjected to solar radiation 13 passing through superstrate 12. Each photovoltaic cell 11 includes a front contact 14 of transparent conductive oxide, a photovoltaic element 18 made of a semiconductor material such as, for example, hydrogenated amorphous silicon, and a back contact 22 of a metal such as aluminum. Photovoltaic element 18 can comprise, for example, a PIN structure. Adjacent front contacts 14 are separated by first grooves 16, which are filled with the semiconductor material of photovoltaic elements 18. The dielectric semiconductor material in first grooves 16 electrically insulates adjacent front contacts 14. Adjacent photovoltaic elements 18 are separated by second grooves 20, which are filled with the metal of back contact 22 to provide a series connection between adjacent front and back contacts. Adjacent back contacts 22 are electrically isolated from one another by third grooves 24.
The thin-film photovoltaic module of FIG. 1 typically is manufactured by a deposition and patterning method. One example of a suitable technique for depositing a semiconductor material on a superstrate is glow discharge in silane, as described, for example, in U.S. Pat. No. 4,064,521. Several patterning techniques are conventionally known for forming the grooves separating adjacent photovoltaic cells, including silkscreening with resist masks, etching with positive or negative photoresists, mechanical scribing, electrical discharge scribing, and laser scribing. Laser scribing and silkscreening methods have emerged as practical, cost-effective, high-volume processes for manufacturing thin-film semiconductor devices, including amorphous silicon photovoltaic modules. Laser scribing has an additional advantage over silkscreening because it can separate adjacent cells in a multi-cell device by forming separation grooves having a width less than 25 micrometers, compared to the minimum practical silkscreening groove width of approximately 125 micrometers. A photovoltaic module fabricated with laser scribing thus has a larger percentage of its surface area actively engaged in producing electricity and consequently has a higher efficiency than a module fabricated by silkscreening. A method of laser scribing the layers of a photovoltaic module is disclosed in U.S. Pat. No. 4,292,092.
Referring to FIG. 1, a method of fabricating a multi-cell photovoltaic module using laser scribing comprises: depositing a continuous film of transparent conductive oxide on a transparent superstrate 12, scribing first grooves 16 to separate the transparent conductive oxide film into front contacts 14, fabricating a continuous film of a photovoltaic semiconductor material on top of front electrodes 14 and in first grooves 16, scribing second grooves 20 parallel and adjacent to first grooves 16 to separate the semiconductor material into individual photovoltaic elements 18, forming a continuous film of metal on elements 18 and in second grooves 20 so that the metal forms electrical connections with front contacts 14, and then scribing third grooves 24 parallel and adjacent to second grooves 20 to separate adjacent back contacts 22.
Complete reliance on laser scribing to pattern photovoltaic modules has not been practical, however, because scribing third grooves 24 to separate back contacts 22 has been found to damage the underlying semiconductor material of photovoltaic elements 18. When the photovoltaic elements are comprised of amorphous silicon, the damage resulting from laser scribing the overlying back contacts includes recrystallization of the amorphous silicon. Such recrystallization tends to create electrical connections between adjacent back contacts, which produces short circuits between paired front and back contacts and substantially reduces the efficiency of the photovoltaic module. Shorting also can result from the laser causing the back contact metal to diffuse into the underlying semiconductor material to form conductive alloys. Consequently, in prior art patterning methods, silkscreening with acid etching must generally be used to form the grooves separating the back contacts and to produce an operable photovoltaic module.
Reliance on silkscreening, however, reduces the active area (i.e., the current producing and collecting area) of the photovoltaic module and therefore reduces its photovoltaic efficiency. Acid etching also requires additional processing steps that significantly decrease output and increase labor costs. Etching third grooves 24 separating back contacts 22 requires the steps of (1) forming a silkscreened pattern over the metal layer to cover the back contacts and expose the desired dividing lines between the back contacts, (2) etching the exposed portions of the metal with acid to form third grooves 24, (3) rinsing the photovoltaic module to remove the acid, (4) applying a solvent to remove the silkscreened pattern, (5) rinsing the photovoltaic module to remove the solvent, and (6) drying the photovoltaic module.
The present invention is intended to eliminate the disadvantages of acid etching by providing a method of forming back contacts on thin-film semiconductor devices, including amorphous silicon photovoltaic modules, by laser scribing the grooves separating the back contacts without damaging the underlying semiconductor material and producing deleterious electrical shorts.
The present invention also is intended to provide a multi-cell photovoltaic module having increased power, efficiency, and reliability through the use of laser scribing to form all the grooves separating adjacent cells.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the method and device particularly pointed out in the appended claims.